Easy separation dual-function Cu2O@LDH@Fe3O4 adsorbent for the removal of Cr(VI) under dark conditions: Experimental and mechanistic study

Designing a magnetic adsorbent material that is both reducible and has strong adsorption properties is significant in reducing Cr(VI) ion pollution. Herein, a 3D multifunctional composite adsorbent for Cr(VI) ions was obtained by using Fe3O4 surface-arrayed magnesium iron layered double hydroxides (Mg/Fe-LDH) micro sheets as a substrate with in situ growth of Cu2O nanoparticles (Cu2O NPs) on their surfaces. Under dark conditions, adsorption capacity of the Cu2O@LDH@Fe3O4 for Cr(VI) was up to 218.82 mg·g−1, surpassing that of Cu2O, Mg/Fe-LDH@Fe3O4, and Mg/Fe-LDH by factors of 1.39, 4.41, and 8.32 respectively. Surprisingly, the removal of Cr(VI) was as high as 76.16 % by the Cu2O@LDH@Fe3O4 (Cinitial = 200 mg/L, pH = 3). Kinetic, isotherm, and thermodynamic results reveal that the removal of Cr(VI) by the Cu2O@LDH@Fe3O4 is consistent with the pseudo-second-order kinetic model of spontaneous monomolecular layer chemisorption. Density functional theory (DFT) calculations show that HCrO4- has a lower energy band gap and is more easily reduced by Cu2O. Therefore, the reason for the adsorbent having more removal effect under acidic conditions was revealed. The XPS and FT-IR experimental mechanism analysis results indicated that Cr(VI) is immobilized on the Cu2O@LDH@Fe3O4 surface through electrostatic interactions, followed by reduction to Cr(III) by Cu2O NPs, which is subsequently adsorbed on the surface of the Cu2O@LDH@Fe3O4. This novel adsorbent structure and unique Cr(VI) removal mechanism provide new ideas for the design of future adsorbents.